The autofocus (hereinafter AF) of imaging devices, such as video cameras and similar cameras, using image detecting elements is generally based on the contrast mode. The contrast mode integrates high frequency components of projected image signals in some range (focus area) among projected image signals (brightness signals) obtained from an image detecting element as an evaluation value and automatically makes a focus adjustment so as to maximize the evaluation value. The contrast mode gives the best focus with a maximum sharpness (contrast) of an image at an image detecting element for image production. However, the contrast mode searches for the best focus while moving focusing lens elements in what is called a hill-climbing mode, which has the disadvantage of a slow rate of focusing.
Accordingly, AF systems and methods wherein the focus state (front focus, back focus and in-focus) of an imaging lens is detected to control the focus by using multiple image detecting elements with different optical path lengths have been proposed to resolve the drawback of such a contrast mode. These AF systems detect the focus state at the image detecting plane where an image detecting element for image production is located to control focus by positioning focus state detecting elements equidistant in front of and behind positions conjugate with the image detecting element for image production. The two focus state detecting elements obtain quantitative evaluation values indicative of the focus state at each of the pair of image detecting elements. A comparison of the magnitude of those evaluation values provides information about the focus state at the image detecting plane. However, conventional AF systems have the drawback that if the deviation of focus is too large, the difference in the evaluation values obtained from the two focus state detecting elements disappears, so that the focus state cannot be detected.
The present invention relates to an AF system that controls the focus of an imaging lens by a contrast mode over a wide range of focus by using two pairs of focus state detecting elements to indicate the focus state at an image detecting plane.